1. Field of the Invention
The present invention relates to a motional load driver and, particularly, to a motional load driver for driving a motional load such as a dynamic loudspeaker while reducing distortions thereof and, more particularly, to a motional load driver which can eliminate the adverse influences of a ground potential difference in circuits in a driving operation.
2. Description of the Prior Art
A conventional motional load driver comprises a motional feedback (MFB) circuit for eliminating distortions due to the transient response of a vibration system of a dynamic loudspeaker (to be simply referred to as a loudspeaker hereinafter) using a pressure sensor, a temperature sensor, a microphone, or the like. The MFB circuit negatively feeds back a motional voltage generated by the motional impedance of the loudspeaker to the input side of a power amplifier. Instead of using the above-described sensors and the like, a bridge circuit may be used. According to this method, the bridge circuit detects a motional voltage generated at a terminal of a voice coil of a loudspeaker (refer to a monthly magazine "Radio Technique" published in Japan; Oct. Issue and Nov. Issue in 1984, and Feb. Issue in 1985).
In the MFB circuit using the above-described sensors and the like, however, a feedback quantity is limited depending on the performance of the sensor or the like, e.g., an increase in phase revolution of a detection output of the sensor (if the feedback quantity is set large, the MFB circuit starts to oscillate). For this reason, only a small distortion reducing effect of the loudspeaker can be expected.
On the other hand, in the MFB circuit using the bridge circuit, the circuit arrangement for detection becomes complicated.
As described above, the conventional drivers using the MFB circuits must detect a motional voltage. Therefore, it is impossible to sufficiently reduce distortions of a loudspeaker.
Therefore, in consideration of the fact that transient response distortions in a motional load driving operation are caused because the motional load includes impedance components, other than the motional impedance, such as DC resistance components and inductance components, the present applicant previously proposed a motional load driver which could eliminate the distortions due to the transient response by performing negative output impedance driving so as to cancel the impedance components other than the motional impedance, which is disclosed in Japanese Pat. Application No. 62-145738 corresponding to U.S. Ser. No. 199,479. FIG. 5 shows the arrangement of the driver.
Referring to FIG. 5, an input terminal 1 which receives an input voltage Vi is connected to the inverting input terminal of an operational amplifier 2 (power amplifier), through a resistor R1. The non-inverting input terminal of the operational amplifier 2 (power amplifier) is grounded. The output terminal of the operational amplifier 2 is connected to the connection point of the resistor R1 and the inverting input terminal of the amplifier 2 through a resistor R3, and at the same time is grounded through a series circuit of a load 3 (impedance ZL) as a loudspeaker and a resistor Rs. The connection point of the load 3 and the resistor Rs is connected to the connection point of the inverting input terminal and the resistors R1 and R3 through a series circuit of a gain-A amplifier 4 (servo amplifier) and a resistor R2. The amplifier 4 and the resistor Rs constitute a load impedance detecting circuit (or a load resistance detecting circuit). The main part of the present invention corresponds to the part constituted by the amplifier 4.
In this case, assuming that the voltage across the load 3 is Vo, then its transmission characteristic is represented by: EQU -Vo/Vi=(R3/R1).multidot.[1/{1+(Rs/ZL).multidot.(1-A.multidot.R3/R2)}]. . .(1)
Therefore, an output impedance (drive impedance) can be given by: EQU Zo=Rs(1-A.multidot.R3/R2) . . . (2)
If A.multidot.R3/R2&gt;1 in equation (2), impedance components other than the motional impedance of the load impedance can be canceled by setting the output impedance Zo to be a predetermined negative value. In order to satisfy this condition, however, ground potentials E1 and E2 at the input and output sides must be equal to each other.
However, even if the motional load driver is arranged on a single substrate, it is difficult to ground the input and output sides to a single ground point. For this reason, the input and output sides are grounded at different ground points. In this case, the impedance of a ground pattern on the substrate is not zero, but a very low impedance is included in the ground pattern. As a result, a potential difference occurs between the input and output side ground points. Furthermore, in the above arrangement, a variation in motional impedance of the load 3 is detected by the detecting resistor Rs as a voltage variation, and this voltage variation is fed back to the inverting input terminal of the operational amplifier 2 through the amplifier 4. For this reason, the circuit in FIG. 5 is influenced by the potential difference between the input and output ground points.
This phenomenon will be described by using the following equation. When the ground potentials E1 and E2 on the input and output sides are different from each other, the output impedance Zo can be represented by: EQU Zo=Rs.multidot.(1-A.multidot.R3/R2).multidot.[1-(R1/R2).multidot.A.multidot .(Vg/Vi)].sup.-1. . . (3)
for Vg=E2-E1.
As is apparent from equation (3), the output impedance Zo varies in accordance with the potential difference Vg. Distortions are generated by the influences of this potential difference Vg. In addition, the circuit operation becomes unstable.
As described above, according to the arrangement in FIG. 5, the motional load can be driven with small distortions without detecting the motional voltage. However, if a potential difference occurs between the input and output ground points when an output current value is fed back to the input side to obtain a negative output impedance, the output impedance value varies, and hence accurate cancelation cannot be realized.